I would have thought that with a track record of the issue at hand and that it was bought to the attention and attempted to be rectified by Holden that they had in some way already accepted responsiblilty for the issue at hand.
I am surprised consumer affair backed off so easily. I had a run in with Holden with my car when I bought it too. The paint on the front bumper started peeling and was Arden Blue underneath!!!!
They accused me of all sorts of things such as getting a 2nd hand bumper off another car etc...In the end I managed to get them to fix it at their cost, but it was a struggle.
If it was me I would pursue it. FYI, there is an M32 gearbox on UK eBay for GBP 550 or AUD$1100 that might be what your looking for. I'm not sure if yours is the 5 or 6 speed but it would be well cheaper than $7000 to get over and fitted....
http://cgi.ebay.co.uk/ASTRA-MARK-5-V...d=p3286.c0.m14
try them they might just be able to work out shipping with these guys
www.wedelivertheworld.co.uk